Electric contact



Sept. 13, 1960 R. E. SEMPLE ET AL 2,952,827

ELECTRIC CONTACT Filed Nov. 13. 1956 F G. 2. T F 6. 3.

/4 /8 /O W 4L 'NI n il 'v I ilu i lf' li n 20 /y /6 /3 241 2F24 F/G. 4.

)A f 22 22 SO 20 IN V EN TORS. ROBERT E. SEMPLE ROBERT L. P/TZER A T TOR/VE YS United States Patent C) "ce ELECTRIC CONTACT Robert E. Semple, Temple City, and Robert L. Pitzer, Arcadia, Calif., assignors to Edclit Instruments, Duarte, Calif., a corporation of California Filed Nov. 13, 1956, Ser. No. '621,552

5 Claims. (Cl. SSS- 183) This invention relates to an improved electric contact for adjustable resistors ofthe 4type having a wire coil and a resilient contact engaging along a surface of the coil.

The operation of resistor devices is well known. A volt-age source is connected to the end terminals of a resistance element, and an output circuit is connected between one end terminal and a movable contact to give a voltage-dividing action.

The electrical contact often consists of a spring member connected to a slider. The spring member has a portion thereof in contact with a coil winding mounted in the potentiometric instrument casing. It the slider is moved at a high rate of speed, the spring member may oscillate. A space exists between e-ach Winding of the coil, so that the spring member actually rides over small corrugations. If the movement over the corrugations is such as to approach the resonant frequency of the spring, the spring oscillates. These oscillations cause the electrical contact to alternately contact and break contact with the coil, resulting in electrical chat-ter. This chatten or the lbreaking and making of contact with the coil, is undesirable. Chatter can .be eliminated to a certain extent by increasing the mass of the electrical contact. However, if the mass of the contact is increased, the pressure of the contact upon the coil is correspondingly incre-ased.

This pressure causes wear and tear upon the coil, resulti ing in damage to the coil.

I-t is highly desirable, therefore, to damp the movements of the electrical Contact while at the same time keeping the mass of the contact constant. We prevent chatter by applying to a predetermined portion of the electrical contact a coating which is light in Weight and which does not substantially increase the mass of the contact member. The coating is such that the oscillations of the spring caused by the sliding movement of the slider yare substantially eliminated, hence, eliminating chatten Because of the llight weight of the coating, the pressure of the electrical contact upon the coil is not increased to any extent and therefore there is no wear and tear upon the coil.

Particular coating materials which etectively damp movements of the electrical contact are those low speciic .gravity materials having a low modulus of elasticity, a high elastic limit and high energy absorption. Coating materials having this combination of properties are particularly found among plastic Imaterials such as natural rubbers, synthetic rubbers and certain vinyl jresins. Among the synthetic rubbers, coating materials having the proper physical properties may be selected from the Thiokol rubbers, a term applied -to lthe linear polymers 'chielly produced from the reaction of ethyl dichloride and sodium tetrasuliide,-and neoprene, a term applied to the polymers formed by the polymerization of chloroprene. Suitable natural and synthetic rubbers may be applied as a coating in both natural and artificial dispersions, herein referred to as latex.

As previously discussed, the property of low specilic gravity is required for the material so that when applied to 'portions of the instrument.

the contact the weight of the coating does not I'add substantially to the mass acting against the electrical coil. As used, 'the term low specific gravity is intended to be comparative with respect to the density ofthe metals from which the conductive contact arm is necessarily made. For example, as compared to a specific gravity on 'the order of 7 to 8 for conductive metals commonly used 'for contact arms, the specific gravity of a natural rubber is on the order of 1 and the specific gravity of 'a synthetic rubber such as neoprene is 1.25.

The requirement of a low modulus of elasticity for the coating material permits effective damping without effect upon the normal spring characteristics of the contact arm. Therefore, it is desired that the modulus of elasticity of the coating material `be substantially lower than the modulus of elasticity of the material from which the contact armis formed, the latter modulus 'typically varying from 16x106 Ito 18X 106 p.s.i.

rThe effectiveness of the damping action of the coating material is determined bythe degree of energy absorption of the material. The hysteresis index is a yfactor which provides `a quantitative basis for comparison of the relative energy absorption of various materials. For example, among plastic materials such as those previously enumerated, the hysteresis index measured in shear at 60 cycles per second at 30 C. in units of megagrarns X cm.1 X secr2 is about 3 for a natural rubber and is as rhigh as 14 vfor some synthetic rubbers. A plastic material having 'a hysteresis index of 2 or more at a temperature of 30 C. in the 'units specilied above is considered as having a high energy absorption suitable for use as a coating material according to the invention.

The plastic material may be applied by painting or spreading it on the portion of the electrical contact which is not in contact with the coil. These materials are their )wn adhesive in many cases.

The advantages of our new electrical contact will become more apparent as the description is read in conjunction with 'the accompanying drawings, in which:

Fig. l is ya side elevational schematic view, partly in section, showing important components of ya potentiometer;

Fig. Z is a sectional view taken along lines 2-2 of Fig. l;

Fig. 3 is a plan view of one form of electrical contact;

Fig. 4 is an enlarged sectional view taken along lines 4-4 odi Fig. 3;

Y Fig. 5 is a side elevational view of a second type of improved electrical contact; and

Fig. 6 is a view showing an electrical contact with the coating material applied to the under side thereof.

The instrument shown in Fig. 1 includes the essential Within a casing 10 there is mounted a coil support member 12 upon which is wound an electrical conducting coil 13 of many turns. A slider 14 is mounted for slidable movement longitudinally within the casing 10. Slider 14 is moved by a shaft `16. Shaft 16 extends through an opening in the side yof casing 10.

The slider 14 has a groove formed in the upper portion thereof. A guide rod 18 which is mounted in the upper portion of casing 10 ts within the groove. The guide rod 18 prevents rotational movement of the slider 144as the slider is moved longitudinally within `the casing 10 by shaft 16.

tinually in contact with a shorting bar 24 mounted within casing 10.

In the operation of the instrument the coil 13 wound about coil bar 12 is connected between a pair of terminals to which avoltage is applied. The electrical contact, including coilv contact 20 and continuity bar 22 acts as a variable voltage tap. As the electrical contact is moved along the coil the voltage across a terminal of lthe coil and the contact point of contact 20 is varied. A difficulty which arises as the slider 14 is moved too rapidly is that the coil contact 20 and the continuity bar ,contact 22 begin -to oscillate, causing them to make and break contact, resulting in chatten The amount of chatter depends upon the resonant frequency of the contacts 20 and 22, and the number of turns of the wire 13. For example, if contacts 20 and 22 have a resonant frequency of 1500 cycles per second, and the coil has 1000 turns per inch, the chatter will be excessive if the slider 14 is moved in excess of 11/2 inches per second. The resonant frequency of the contacts can be increased by increasing the mass of the contacts. However, the pressure upon the coils will then be excessive and free movement of contacts 20 and 22 will be hindered. Also, this excessive pressure can damage the coils. We eliminate the undesired chatter without increasing the mass of the spring members Iby applying special types of coating material to the electrical contacts. 3 and 4 show one `form of improved electrical contact which can be included in the instrument of Figs. 1 and 2. A coating 30 is applied to any portion of the electrical contacts which is not in contact with the coil or the shorting bar. The areas of the spring which will show maximum rate of change of deflection when the contact is displaced should be covered. We prefer to cover at least the aft two-thirds of the contacts 20 and 22.

Fig. shows a different type of contact from the coil contact shown in Figs. 3 and 4. 'This contact has a V-shaped portion which is adapted to contact the coil in an adjustable resistor. The coating 30 is applied on the upper portion of the contact. lt is important that the trough of the V be uncovered, as to cover this por- ;tion with the coating would interfere with the proper operation of -the instrument.

l If sufficient space exists between the. under side of the coil contact and the coil, the coating may be placed on the under side. For example, Fig. 6, which shows the shaft 16, the slider 1'4 and the coil bar 12 with a coil 13 wound thereabout, also includes an electrical contact 60 which has a depending portion which contacts the coil 13. Since sufficient space exists between the contact 60 and the coil 12, the coating 30 is shown applied to 4the under side of the contact y60.

When 'a damping material such as one of the synthetic Thiokol rubbers or latex vis spread or painted upon a portion of the electrical contact, the undesirable chatter is eliminated and the pressure of the contact upon the coil is not increased in amount so as to damage or J exert too much pressure upon the coil. 'Ihere is no ynoticeable effect on the spring characteristics of the contact. 'I'his is so because the elastic modulus of this The material also has a the plastic material does not interfere with the normal characteristics of the spring.

The elastic modulus of spring material commonly -used in electrical contacts ranges from 16x106 p.s.i. to 18x10 p.s.i. 'I'he elastic modulus of the damping ma- Figs.

. 4 terial used ranges from 100 p.s.i. to 125 p.s.i. at 100 percent elongation.

The elastic limit of spring material commonly used in electrical contacts is 135,000 p.s.i. The damping material, on the other hand, can be elongated more than 325 percent.

For optimum effect, the coating material should be at least as thick as thebase metal of the spring member.

We claim:

l.'An adjustable device comprising an electric coil, a

support member spaced from the coil and movable relative to the coil, a resilient conductive member attached to the support member and having a free end extending from the support member to engage a surface of the electric coil, and a low specilic gravity plastic coating material having a high degree of energy absorption and a modulus of elasticity less than the modulus of elasticity of the resilient conductive member, the coating material covering' the portion of the surface area of the resilient conductive member out of contact with the electrical coil, whereby vibratory motions of the conductive member 'are damped. v 2. An adjustable resistor comprising a resistance Wire coil, a support member spaced from the coil and rnov`- able relative to the coil, a resilient conductive member ,attached to the support member and having a free end extending from the support member to engage a surface of the resis-tance wire coil and a plastic coating material having va specific gravity less than 2, a hysteresis index of at least 2 megagrams per centimeter per sec.2 at a temperature of 30 C., and a modulus of elasticity less than the modulus of elasticity of the resilient conductive member, the coatingmaterial covering a portion of the surface area of the resilient conductive member out of contact with the electrical coil, whereby vibratory motions of the conductive member are damped.

3. Apparatus in accordance with claim 2 wherein the coat-ing material is a synthetic rubber.

4. Apparatus in accordance with claim 2 wherein twothirds of one surface of the resilient conductive member is coated with a coating material.

5. An adjustable resistor comprising an electrical coil, a support member spaced from the coil and movable relative to the coil, a resilient conductive member lattached to the support member and including an elonlgated portion extending from the support member and a contacting portion engaging a surface of the electric coil, and a low specic gravity plastic coating material having a high degree of energy absorption and a modulus of elasticity less than the modulus of elasticity of the resilient conductive member, the coating material joined to cover a part of the surface area of the elongated portion of the conductive member without covering the contacting portion, whereby vibratory motions of the conductive member are damped.

References Cited in the file of this patent UNITED STATES PATENTS 

